The ultrasonic quality control of double-layer bonded construction of a workpiece is of great industrial importance. An example of such a workpiece is a tube made of low carbon or low alloyed steel (parent metal) with a thin layer of stainless steel (clad metal) clad on the tube's internal or external surface.
Quality control and control of clad metal thickness are usually solved by means of direct, single or dual ultrasonic probes, as disclosed, e.g., in U.S. Pat. No. 5,349,860, filed Sep. 27, 1994, entitled "Apparatus for Measuring the Thickness of Clad Material." The apparatus provides control of the bonding quality at the boundary of two metals by means of registration of the amplitude of the echo signal reflected from the boundary, however with low sensitivity, because of the low amplitude of echo signals reflected from the boundary, making it difficult to detect defects in the bonding. The apparatus further provides control of the clad metal thickness, but with low precision because of the weak level (amplitude) of the ultrasonic signal reflected or refracted from the bonded interface. In fact, the angle of propagation of the echo signal B in a material under control is determined by the parameters of the transmitter and by the receiver's angle-type probes, while the angle of the echo signal C is additionally determined by the clad metal thickness. Therefore, the time of propagation of the echo signal B from the transmitter to the receiver's piezo crystals (t.sub.)) is directly proportional to the total thickness of a material (mother clad metal), whereas the time of propagation (t.sub.1) of the echo signal C is not directly proportional to the parent metal thickness. Hence, the difference in the propagation times of the echo signals B and C is not directly proportional to the clad metal thickness, but is considerably smaller than its real value. This leads to a large error in measuring the clad metal thickness.